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[RFC][PATCH -next 2/2] printk: set may_schedule for some of console_trylock callers

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 console_unlock() allows to cond_resched() if its caller has
set `console_may_schedule' to 1 (this functionality present
since commit 'printk: do cond_resched() between lines while
outputting to consoles').

The rules are:
-- console_lock() always sets `console_may_schedule' to 1
-- console_trylock() always sets `console_may_schedule' to 0

However, console_trylock() callers (among them is printk()) are
not necessarily executing in atomic contexts, and some of them
can cond_resched() in console_unlock(). So console_trylock()
can set `console_may_schedule' to 0 only if cond_resched() is
invalid in the current context, and set it to 1 otherwise.

The patch also drops explicit preempt_disable()/preempt_enable()
calls in vprintk_emit().

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
---
 kernel/printk/printk.c | 13 ++++---------
 1 file changed, 4 insertions(+), 9 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 5cc0ce6..d0f5a73 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1792,20 +1792,12 @@ asmlinkage int vprintk_emit(int facility, int level,
 	if (!in_sched) {
 		lockdep_off();
 		/*
-		 * Disable preemption to avoid being preempted while holding
-		 * console_sem which would prevent anyone from printing to
-		 * console
-		 */
-		preempt_disable();
-
-		/*
 		 * Try to acquire and then immediately release the console
 		 * semaphore.  The release will print out buffers and wake up
 		 * /dev/kmsg and syslog() users.
 		 */
 		if (console_trylock_for_printk())
 			console_unlock();
-		preempt_enable();
 		lockdep_on();
 	}
 
@@ -2138,7 +2130,10 @@ int console_trylock(void)
 		return 0;
 	}
 	console_locked = 1;
-	console_may_schedule = 0;
+	console_may_schedule = !(oops_in_progress ||
+			in_interrupt() ||
+			irqs_disabled() ||
+			in_atomic());
 	return 1;
 }
 EXPORT_SYMBOL(console_trylock);
-- 
2.7.0

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[RFC][PATCH -next 0/2] cond_resched() some of console_trylock callers Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> - 2016-01-14 06:00 +0100
  [RFC][PATCH -next 1/2] printk: move can_use_console out of console_trylock_for_printk Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> - 2016-01-14 06:00 +0100
    [PATCH 1/2] printk: move can_use_console out of console_trylock_for_printk Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> - 2016-01-14 07:00 +0100
  [RFC][PATCH -next 2/2] printk: set may_schedule for some of console_trylock callers Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> - 2016-01-14 06:00 +0100

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